Document that end users shouldn't be running `git` commands manually where nbgitpuller clones

nbgitpuller expects to have full ownership of the cloned git repository. If the user is manually running `git` commands there, it will eventually break. This is an *essential* complexity of nbgitpuller - its goal is to abstract away git usage from the end user, so it makes assumptions about how git is used. Many things a user can do (particularly switching branches) breaks these assumptions.

If the user wants to run `git` commands locally, they should make a clone with `git clone`.

## Definition of done

- [ ] This information is prominently documented